Light emitting element

ABSTRACT

It is an object of the present invention to provide a light-emitting element having a structure in which the drive voltage is relatively low. Further, it is an object of the invention to provide a highly reliable light emitting device by alleviating the stress to the light emitting layer. Further, it is another object of the invention to provide a light emitting element having a structure in which increase in the drive voltage over time is small. It is an object of the present invention to provide a display device in which the drive voltage is low and increase in the drive voltage over time is small and which can withstand long-term use. In a light emitting element, a layer in contact with an electrode serves as a hole generating layer such as an organic compound layer containing a P-type semiconductor or an electron accepting material, a light emitting layer is provided between hole generating layers, an electron generating layer is formed between the hole generation layer on the cathode side and the light emitting layer.

TECHNICAL FIELD

The present invention relates to a light-emitting element in which athin film containing a light-emitting material is interposed betweenelectrodes and which emits light when current is applied. Moreover, thepresent invention relates to a display device and an electronicappliance which use the light-emitting element.

BACKGROUND ART

A display using a thin film light-emitting element of aself-light-emitting type, which emits light by itself when current isapplied, has been extensively developed.

Such a thin film light-emitting element emits light by connecting anelectrode to a single-layer or multilayer thin film formed using one orboth of an organic compound and an inorganic compound when current isapplied. Such a thin film light-emitting element is expected to reducethe power consumption, occupy smaller space, and increase thevisibility, and the market is also expected to expand further.

It has become possible to manufacture an element which emits light moreefficiently than before by dividing the function for each layer of alight-emitting element having a multilayer structure (for example, seeReference 1: Applied Physics Letters, Vol. 51, No. 12, 913-915 (1987) byC. W. Tang et al.).

A thin film light-emitting element having a multilayer structure has alight-emitting stack provided between an anode and a cathode. Thelight-emitting stack includes a hole-injecting layer, ahole-transporting layer, a light-emitting layer, anelectron-transporting layer, an electron-injecting layer, and the like.Among these layers, all the hole-injecting layer, the hole-transportinglayer, the electron-transporting layer, and the electron-injecting layermay not necessarily be used depending on the element structure.

The hole-injecting layer in the light-emitting stack as above is formedwith a material which can inject holes relatively easily from a metalelectrode into a layer mainly containing an organic compound. Theelectron-transporting layer in the light-emitting stack is formed with amaterial that is superior in electron-transporting properties. Thus,each layer in the light-emitting stack is formed by selecting a materialsuperior in each function.

However, a material mainly containing an organic compound, to whichelectrons can be injected relatively easily from an electrode, or amaterial mainly containing an organic compound which can transportelectrons at a predetermined mobility or more is very limited. As isclear from the limitation on the material, the injection of theelectrons from the electrode into the layer mainly containing theorganic compound is originally rare to occur. Thus, the drive voltage ishigh. Further, an experiment shows that an element of higher drivevoltage has higher drive voltage over time.

DISCLOSURE OF INVENTION

Consequently, it is an object of the present invention to provide alight-emitting element having a structure in which the drive voltage isrelatively low. Further, it is another object of the invention toprovide a light emitting element having a structure in which increase inthe drive voltage over time is small.

Further, it is an object of the present invention to provide a displaydevice in which the drive voltage is low and increase in the drivevoltage over time is small and which can withstand long-term use.

Further, it is also an object of the present invention to improve thereliability of a light emitting element.

According to the present invention, a layer in contact with either ofelectrodes (a first electrode, a second electrode) in a light-emittingelement is a hole-generating layer such as a layer containing a P-typesemiconductor or an organic compound layer containing a material havingelectron-accepting properties, a light-emitting layer is providedbetween the hole-generating layers, and an electron-generating layer isformed between the hole-generating layer on the second electrode sideand the light-emitting layer. This makes it possible to suppress thedrive voltage. Further, increase in the drive voltage over time can besuppressed due to the lower drive voltage.

A light emitting element of one aspect of the structure of the presentinvention is provided with a first electrode and a second electrode; afirst layer and a second layer each containing an organic material andan inorganic material; a third layer containing a light emittingmaterial; and a fourth layer generating electrons. The first layer is incontact with the first electrode, the second layer is in contact withthe second electrode, the third layer is provided between the firstelectrode and the second electrode with the first layer and the secondlayer respectively therebetween, and the fourth layer is providedbetween the third layer and the second layer.

A light emitting element having a structure of another aspect of thepresent invention is provided with a first electrode and a secondelectrode; a first layer and a second layer each generating holes; athird layer containing a light emitting material; and a fourth layergenerating electrons. The first layer is in contact with the firstelectrode, the second layer is in contact with the second electrode, thethird layer is provided between the first electrode and the secondelectrode with the first layer and the second layer respectivelytherebetween, and the fourth layer is provided between the third layerand the second layer.

A light emitting element having a structure of another aspect of thepresent invention is provided with a first electrode and a secondelectrode; a first layer and a second layer each containing a P-typesemiconductor; a third layer containing a light emitting material; and afourth layer containing an N-type semiconductor. The first layer is incontact with the first electrode, the second layer is in contact withthe second electrode, the third layer is provided between the firstelectrode and the second electrode with the first layer and the secondlayer respectively therebetween, and the fourth layer is providedbetween the third layer and the second layer.

A light emitting element having a structure of another aspect of thepresent invention is provided with a first electrode and a secondelectrode; a first layer and a second layer each containing a firstorganic compound and a material which accepts electrons of the firstorganic compound; a third layer containing a light emitting material;and a fourth layer containing a second organic compound and a materialwhich donates electrons to the second organic compound. The first layeris in contact with the first electrode, the second layer is in contactwith the second electrode, the third layer is provided between the firstelectrode and the second electrode with the first layer and the secondlayer respectively therebetween, and the fourth layer is providedbetween the third layer and the second layer.

Further, in the above structure of a light emitting element according tothe invention, the difference between the molar ratio of the electronaccepting material to a first organic compound in the second layer andthe molar ratio of the electron accepting material to the first organiccompound in the first layer is preferably in the range of 80% of themolar ratio of the electron accepting material to the first organiccompound in the second layer and also in the range of 80% of the molarratio of the electron accepting material to the first organic compoundin the first layer. More preferably, the difference between the molarratio of the electron accepting material to the first organic compoundin the second layer and the molar ratio of the electron acceptingmaterial to the first organic compound in the first layer is in therange of 40% of the molar ratio of the electron accepting material tothe first organic compound in the second layer and also in the range of40% of the molar ratio of the electron accepting material to the firstorganic compound in the first layer.

Further, in the above structure of a light emitting element according tothe invention, the thickness of each of the first layer and the secondlayer is 30 nm to 1 μm.

Further, in the above structure of a light emitting element according tothe invention, the thickness of the second layer is 50% to 150% of thethickness of the first layer, and the thickness of the first layer is50% to 150% of the thickness of the second layer.

In a light-emitting element having a structure according to the presentinvention, the drive voltage can be lowered. Further, increase in thedrive voltage over time can be suppressed due to the lower drivevoltage.

Further, a display device in which the drive voltage and increase in thedrive voltage over time is small and which can withstand long-term usecan be provided.

Still further, the stress in the light emitting layer can be alleviated.Accordingly, the reliability of the light emitting element can beimproved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a light-emitting element according to the presentinvention;

FIG. 2 shows a light-emitting element according to the presentinvention;

FIG. 3 shows a light-emitting element according to the presentinvention;

FIG. 4 shows a light-emitting element according to the presentinvention;

FIGS. 5A to 5E show a process for manufacturing a thin filmlight-emitting element according to the present invention;

FIGS. 6A to 6C show a process for manufacturing a thin filmlight-emitting element according to the present invention;

FIGS. 7A and 7B show an example of a structure of a display deviceaccording to the present invention;

FIGS. 8A and 8B show a top view and a cross-sectional view of alight-emitting device according to the present invention;

FIGS. 9A to 9E show examples of electronic appliances to which thepresent invention can be applied;

FIGS. 10A to 10C show examples of structures of a display deviceaccording to the present invention;

FIGS. 11A to 11F show examples of a pixel circuit of a display deviceaccording to the present invention;

FIG. 12 shows an example of a protective circuit of a display deviceaccording to the present invention;

FIG. 13 is a graph showing the voltage-luminance characteristic of anelement in Embodiment 1;

FIG. 14 is a graph showing the voltage-current characteristic of anelement in Embodiment 1;

FIG. 15 is a graph showing the voltage-luminance characteristic of anelement in Embodiment 2;

FIG. 16 is a graph showing the current density-luminance characteristicof an element in Embodiment 2;

FIG. 17 is a graph showing the voltage-current characteristic of anelement in Embodiment 2;

FIG. 18 is a graph showing the change of voltage of an element over timein Embodiment 2; and

FIG. 19 is a graph showing the change of luminance of an element overtime in Embodiment 2.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiment Modes and Embodiments will be hereinafter described withreference to the drawings. However, since the present invention can becarried out in many different modes, it is to be understood by thoseskilled in the art that the modes and details can be modified withoutdeparting from the scope of the present invention. Therefore, thepresent invention is not construed as being limited to the descriptionof the following Embodiment Modes and Embodiments.

Embodiment Mode 1

In the present embodiment mode, structures of a light-emitting elementof the present invention will be described with reference to FIGS. 1 and2. In a light-emitting element according to the present invention, alight-emitting layer 104 containing a light-emitting material and anelectron-generating layer 105 are stacked, and the light-emitting layer104 and the electron-generating layer 105 are provided between a firsthole-generating layer 102 and a second hole-generating layer 103. Thefirst hole generating layer 102 is in contact with a first electrode,and the second hole generating layer 103 is in contact with a secondelectrode. The first hole-generating layer 102 and the secondhole-generating layer 103 are further provided between the firstelectrode 101 and the second electrode 106, and stacked over aninsulator 100 such as a substrate or an insulating film. Over theinsulator 100 such as the substrate or the insulating film, the firstelectrode 101, the first hole-generating layer 102, the light-emittinglayer 104, the electron-generating layer 105, the second hole-generatinglayer 103, and the second electrode 106 are stacked in order (FIG. 1).Alternatively, the order may be opposite: the second electrode 106, thesecond hole-generating layer 103, the electron-generating layer 105, thelight-emitting layer 104, the first hole-generating layer 102, and thefirst electrode 101 are stacked in order (FIG. 2). Note that, in thisembodiment mode, when voltage is applied so as to make a light emittingelement emit light, the electrode applied with higher potential is thefirst electrode, and the electrode applied with lower potential is thesecond electrode.

The first hole-generating layer 102 and the second hole-generating layer103 are each formed with the same material using a layer containing bothof a hole-transporting material and an electron-accepting material whichcan receive electrons from the hole-transporting material, a P-typesemiconductor layer, or a layer containing a P-type semiconductor isused. The thickness of the first hole-generating layer 102 and thesecond hole-generating layer 103 is preferably 30 nm to 1 μm. Further,it is preferable that the thicknesses of the first hole-generating layer102 and the second hole-generating layer 103 be almost the same. Thethickness of the second hole-generating layer is not to be thicker orthinner than the first hole-generating layer by more than 50%.Meanwhile, the thickness of the first hole-generating layer is not to bethicker or thinner than the second hole-generating layer by more than50%. In other words, the thickness of the second hole generating layeris 50% to 150% of the thickness of the first hole generating layer whilethe thickness of the first hole generating layer is 50% to 150% of thesecond hole generating layer. Letting X denote the thickness of thefirst hole generating layer and Y denote the thickness of the secondhole generating layer, 0.5≦(Y/X)≦1.5 and 0.5≦(X/Y)≦1.5 are satisfied.

Further, the first hole generating layer 102 and the second holegenerating layer 103 are preferably formed with the same materials usinga hole transporting material and an electron accepting material whichcan receive electrons from the hole transporting material. Thedifference between the molar ratio of the electron accepting material tothe hole transporting material in the second hole generating layer andthe molar ratio of the electron accepting material to the holetransporting material in the first hole generating layer is preferablyin the range of 80% of the molar ratio of the electron acceptingmaterial to the hole transporting material in the second hole generatinglayer and also in the range of 80% of the molar ratio of the electronaccepting material to the hole transporting material in the first holegenerating layer. More preferably, the difference between the molarratio of the electron accepting material to the hole transportingmaterial in the second hole generating layer and the molar ratio of theelectron accepting material to the hole transporting material in thefirst hole generating layer is in the range of 40% of the molar ratio ofthe electron accepting material to the hole transporting material in thesecond hole generating layer and also in the range of 40% of the molarratio of the electron accepting material to the hole transportingmaterial in the first hole generating layer.

Accordingly, letting A denote the molar ratio of the electron acceptingmaterial to the hole transporting material in the first hole generatinglayer, and B denote the molar ratio of the electron accepting materialto the hole transporting material in the second hole generating layer;(−0.8×A)≦B−A≦(0.8×A) and (−0.8×B)≦B−A≦(0.8×B) are to be satisfied.

As the hole-transporting material, for example, an aromatic aminecompound (having a bond of a benzene ring with nitrogen), phthalocyanine(abbreviated to H₂Pc), or a phthalocyanine compound such as copperphthalocyanine (abbreviated to CuPc) or vanadyl phthalocyanine(abbreviated to VOPc) can be used. The aromatic amine compound is, forexample, 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl (abbreviatedto α-NPD), 4,4′-bis[N-(3-methylphenyl)-N-phenyl-amino]-biphenyl(abbreviated to TPD), 4,4′,4″-tris(N,N-diphenyl-amino)-triphenylamine(abbreviated to TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenyl-amino]-triphenylamine(abbreviated to MTDATA), or4,4′-bis(N-(4-(N,N-di-m-tolylamino)phenyl)-N-phenylamino)biphenyl(abbreviated to DNTPD). As the electron-accepting material which canreceive electrons from the hole-transporting material, for example,molybdenum oxide (MoOx), vanadium oxide,7,7,8,8,-tetracyanoquinodimethane (abbreviated to TCNQ),2,3-dicyanonaphtoquinone (abbreviated to DCNNQ),2,3,5,6-tetrafluoro-7,7,8,8,-tetracyanoquinodimethane (abbreviated toF4-TCNQ), or the like is used. The electron-accepting material which canreceive electrons is selected in accordance with the combination withthe hole-transporting material. Further, metal oxide such as molybdenumoxide (MoOx), vanadium oxide, ruthenium oxide, cobalt oxide, nickeloxide, or copper oxide can be used as the P-type semiconductor.

Further, instead of the first hole generating layer 102 and the secondhole generating layer 103, a first layer and a second layer, each ofwhich is formed with a layer containing an organic compound such as theabove described hole transporting material and an inorganic material ortwo or more inorganic materials selected from zinc oxide, indium oxide,tin oxide, antimony oxide, tungsten oxide, indium nitride, tin nitride,antimony nitride, a tungsten nitride, and molybdenum nitride. Thethickness of the first layer and the second layer is preferably 30 nm to1 μm. Further, the thicknesses of the first layer and the second layerare preferably almost the same, and the thickness of the second layer isnot to be thicker or thinner than that of the first layer by more than50%. Meanwhile, the thickness of the first layer is not to be thicker orthinner than that of the second layer by more than 50%. In other words,the thickness of the second layer is 50% to 150% of the thickness of thefirst layer, and the thickness of the first layer is 50% to 150% of thethickness of the second layer. Further, the first layer and the secondlayer are preferably formed with respective layers containing the samehole transporting material and the same inorganic material. Thedifference between the molar ratio of the electron accepting material toa first organic compound in the second layer and the molar ratio of theelectron accepting material to the first organic compound in the firstlayer is preferably in the range of 80% of the molar ratio of theelectron accepting material to the first organic compound in the secondlayer and also in the range of 80% of the molar ratio of the electronaccepting material to the first organic compound in the first layer.More preferably, the difference between the molar ratio of the electronaccepting material to the first organic compound in the second layer andthe molar ratio of the electron accepting material to the first organiccompound in the first layer is in the range of 40% of the molar ratio ofthe electron accepting material to the first organic compound in thesecond layer and also in the range of 40% of the molar ratio of theelectron accepting material to the first organic compound in the firstlayer.

As the electron-generating layer 105, a layer containing both of anelectron-transporting material and an electron donating material whichcan donate electrons to the electron-transporting material, an N-typesemiconductor layer, or a layer containing an N-type semiconductor canbe used. As the electron-transporting material, for example, thefollowing can be used; a metal complex having a quinoline skeleton or abenzoquinoline skeleton such as tris-(8-quinolinolato)aluminum(abbreviated to Alq₃), tris(4-methyl-8-quinolinolato)aluminum(abbreviated to Almq₃), bis(10-hydroxybenzo[h]-quinolinolato)beryllium(abbreviated to BeBq₂), orbis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum (abbreviated toBAlq). Besides, a metal complex having an oxazole or thiazole ligandsuch as bis[2-(2-hydroxyphenyl)benzoxazolate]zinc (abbreviated toZn(BOX)₂) or bis[2-(2-hydroxyphenyl)benzothiazolate]zinc (abbreviated toZn(BTZ)₂) can be used. In addition to the metal complex,2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviated toPBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazole-2-yl]benzene(abbreviated to OXD-7),3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole(abbreviated to TAZ),3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole(abbreviated to p-EtTAZ), bathophenanthroline (abbreviated to BPhen),bathocuproin (abbreviated to BCP), or the like can be used. As theelectron donating material which can donate electrons to theelectron-transporting material, for example, alkali metal such aslithium or cesium, magnesium, alkali-earth metal such as calcium, orrare-earth metal such as erbium or ytterbium can be used. The electrondonating material which can donate electrons is selected in accordancewith the combination with the electron-transporting material. Further, ametal compound such as metal oxide can be used as the N-typesemiconductor, and for example zinc oxide, zinc sulfide, zinc selenide,titanium oxide, or the like can be used.

The light-emitting layer 104 containing the light-emitting material isdivided into two types. One of them is a layer in which a light-emittingmaterial to be a luminescence center is diffused in a layer formed witha material having a wider energy gap than the light-emitting material.The other one is a layer consisting of a light-emitting material. Theformer structure is preferred because the concentration quenching isdifficult to occur. As the light-emitting material to be theluminescence center, the following can be employed;4-dicyanomethylene-2-methyl-6-[-2-(1,1,7,7-tetramethyl-9-julolidyl)ethenyl)-4H-pyran(abbreviation: DCJT);4-dicyanomethylene-2-t-butyl-6-[2-(1,1,7,7-tetramethyl-julolidine-9-yl)ethenyl]-4H-pyran;periflanthene;2,5-dicyano-1,4-bis[2-(10-methoxy-1,1,7,7-tetramethyl-julolidine-9-yl)ethenyl]benzene,N,N′-dimethylquinacridone (abbreviated to DMQd), coumarin 6, coumarin545T, tris (8-quinolinolato)aluminum (abbreviated to Alq₃),9,9′-bianthryl, 9,10-diphenylanthracene (abbreviated to DPA),9,10-bis(2-naphthyl)anthracene (abbreviated to DNA),2,5,8,11-tetra-t-butylperylene (abbreviated to TBP), or the like. As thematerial to be a base material in the case of forming the layer in whichthe light-emitting material is diffused, the following can be used; ananthracene derivative such as 9,10-di(2-naphtyl)-2-tert-butylanthracene(abbreviated to t-BuDNA), a carbazole derivative such as4,4′-bis(N-carbazolyl)biphenyl (abbreviated to CBP), or a metal complexsuch as tris(8-quinolinolato)aluminum (abbreviated to Alq₃),tris(4-methyl-8-quinolinolato)aluminum (abbreviated to Almq₃),bis(10-hydroxybenzo[h]-quinolinato)beryllium (abbreviated to BeBq₂),bis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum (abbreviated toBAlq), bis[2-(2-hydroxyphenyl)pyridinato]zinc (abbreviated to Znpp₂), orbis[2-(2-hydroxyphenyl)benzoxazolate]zinc (abbreviated to ZnBOX). As thematerial which can singly constitute the light-emitting layer 104,tris(8-quinolinolato)aluminum (abbreviated to Alq₃),9,10-bis(2-naphtyl)anthracene (abbreviated to DNA), orbis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum (abbreviated toBAlq) or the like can be used.

The light-emitting layer 104 may be formed either in a single-layerstructure or a multilayer structure. A hole-transporting layer may beprovided between the first hole-generating layer 102 and the layer inwhich the light-emitting material of the light-emitting layer 104 isdiffused. Further, an electron-transporting layer may be providedbetween the electron-generating layer 105 and the layer in which thelight-emitting material of the light-emitting layer 104 is diffused.These layers are not necessarily provided. Alternatively, only one ofthe hole-transporting layer and the electron-transporting layer may beprovided. The materials of the hole-transporting layer and theelectron-transporting layer conform to those of the hole-transportinglayer in the hole-generating layer and the electron-transporting layerin the electron-generating layer respectively; therefore, thedescription is omitted here. Refer to the description of those layers.

The first electrode 101 is preferably formed of metal, an alloy, anelectrically conductive compound each of which has high work function(work function of 4.0 eV or more), or a mixture thereof. As a specificexample of the first electrode material, the following can be used: ITO(indium tin oxide), ITO containing silicon, IZO (indium zinc oxide) inwhich zinc oxide (ZnO) is mixed by 2 to 20% into indium oxide, gold(Au), platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr),molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd), ormetal nitride such as TiN. Meanwhile, as the second electrode materialused for forming the second electrode 106, it is preferable to usemetal, alloy, an electrically conductive compound each of which has lowwork function (work function of 3.8 eV or less), or a mixture thereof.As the specific example of the second electrode material, the followingcan be used; an element belonging to group 1 or 2 in the periodic table;alkali metal such as Li or Cs or alkali-earth metal such as Mg, Ca orSr. In addition, alloy containing the above element such as Mg:Ag orAl:Li, a compound containing the above element such as LiF, CsF, orCaF₂, or transition metal containing rare-earth metal can also be used.Further, a stack of the above element and another metal (including analloy) such as Al, Ag, or ITO can be used.

In addition to the first electrode 101, the first hole-generating layer102, the light-emitting layer 104, the electron-generating layer 105,the second hole-generating layer 103, and the second electrode 106; thelight-emitting layer may have a third hole-injecting layer 107 betweenthe first hole-generating layer 102 and the light-emitting layer 104(FIGS. 3 and 4). In this case, the third hole generating layer 107 isdesirably formed with a material different from that of the first holegenerating layer 102, the first hole generating layer 102 is formed withthe same material as the second hole generating layer 103. Further, thethickness of the second hole generating layer 103 is not to be thickeror thinner than that of the first hole generating layer 102 by more than50%. Meanwhile, the thickness of the first hole generating layer 102 isnot to be thicker or thinner than that of the second hole generatinglayer 103 by more than 50%. In other words, the thicknesses of the firsthole generating layer 102 and the second hole generating layer 103 areto be almost the same.

The above material is just an example, and the material can be selectedappropriately by a practitioner as long as the advantage of the presentinvention is obtained.

In the light-emitting element having the above structure according tothe present invention, holes are injected from the secondhole-generating layer 103 into the second electrode when voltage isapplied. In addition, electrons are injected from theelectron-generating layer 105 into the light-emitting layer 104.Further, holes are injected from the first hole-generating layer 102into the light-emitting layer 104. Then, the injected electrons andholes are recombined in the light-emitting layer, and luminescence canbe obtained when the excited light-emitting material returns to theground state. Here, in the light-emitting element according to thepresent invention, the electrons are not injected from the electrodeinto the layer mainly containing the organic compound but injected fromthe layer mainly containing the organic compound into the other layermainly containing the organic compound. The electrons are difficult tobe injected from the electrode into the layer mainly containing theorganic compound. Accordingly, in the conventional light-emittingelement, the drive voltage has increased when the electrons are injectedfrom the electrode into the layer mainly containing the organiccompound. However, since the light-emitting element according to thepresent invention is manufactured without such a process, thelight-emitting element with low drive voltage can be provided. Moreover,it is already known from the experiment that the drive voltage increasesover time more drastically when the light-emitting element has higherdrive voltage; therefore, the light-emitting element according to theinvention with low drive voltage is also a light-emitting element inwhich the increase in the drive voltage over time is small.

As to a light emitting element of the present invention, since the lightemitting layer 104 is provided between the electrodes with layers havingalmost the same stress with the light emitting layer 104 and theelectrodes, on the both sides; thus, the stress in the light emittinglayer can be alleviated. Accordingly, the reliability of the lightemitting element can be improved.

In particular, when a light emitting element has a structure in whichlayers in contact with the electrodes are hole generating layers, andthe light emitting layer is provided between the electrodes, with thehole generating layers on both sides of the light emitting layer, forexample, with the structure in which a hole generating layer is over anelectrode, a light emitting layer is provided thereover, another holegenerating layer is provided thereover, and the other electrode isprovided thereover; the stress on both sides of the light emitting layeris alleviated and the reliability of the light emitting element isimproved. Incidentally, a hole generating layer is a layer whichcontains an organic material and a metal oxide. For example, a layercontaining the above hole transporting material and a molybdenum oxide(MoOx), a zinc oxide, or an indium oxide; or a layer containing theabove hole transporting material and two or more metal oxides selectedfrom a molybdenum oxide (MoOx), a zinc oxide, and an indium oxide ispreferable. Note that another layer may be provided between the holegenerating layer and the light emitting layer.

Embodiment Mode 2

Another embodiment mode of the present invention will be described. Thepresent embodiment mode describes an example of improving thecharacteristic of a viewing angle of a light-emitting element and adisplay device by appropriately adjusting the thicknesses of the firsthole-generating layer 102 and the second hole-generating layer 103.Since the layered structure and the material of the light-emittingelement in the present embodiment mode are the same as those inEmbodiment Mode 1, the description is omitted here. Refer to EmbodimentMode 1. Note that, also in the present embodiment mode, when voltage isapplied so as to make a light emitting element emit light, the electrodeapplied with higher potential is the first electrode, and the electrodeapplied with lower potential is the second electrode.

Light emitted from the light-emitting element includes light directlyemitted from the light-emitting layer 104 and light emitted after beingreflected once or multiple times. The light directly emitted and thelight emitted after being reflected interfere in accordance with therelation between their phases so that they are intensified or attenuatedwith each other. Therefore, the light emitted from the light-emittingelement is light which has been combined as a result of theinterference.

The phase of light reflected when entering a medium having highrefractive index from a medium having low refractive index is inverted.For this reason, in the light-emitting element having the structureshown in Embodiment Mode 1, the phase of light is inverted when thelight is reflected at the interface between the electrode such as thefirst electrode 101 or the second electrode 106 and the layer in contactwith the electrodes. When the light reflected at the electrodeinterferes with the light emitted from the light-emitting layer, in thecase where the optical distance (refractive index×physical distance)between the light-emitting layer and the electrode satisfies (2m−1)λ/4(m is a natural number of 1 or more and λ is a center wavelength of thelight emitted from the light-emitting layer), it is possible to decreasethe change of the spectrum shape which occurs depending on the angle ofviewing a surface from which light is extracted and to increase thecurrent efficiency of the light-emitting element. The current efficiencyshows the luminance with respect to the flowed current. When the currentefficiency is higher, predetermined luminance can be obtained even witha smaller amount of current. Moreover, the deterioration of the elementtends to be little.

Since the reflection is small between films having small refractiveindex difference, the reflections except the reflection at the interfacebetween the electrode and the film in contact with the electrode areignorable. Therefore, in this embodiment mode, attention is paid only tothe reflection between the electrode and the film in contact with theelectrode.

In the case of a light-emitting element in which light is extracted fromthe side of the first electrode 101, the light is reflected at thesecond electrode 106. For this reason, in order to increase the currentefficiency of the light-emitting element and to decrease the change ofthe spectrum shape which occurs depending on the angle of viewing thesurface from which the light is extracted, the optical distance(refractive index×physical distance) from the light-emission position tothe surface of the second electrode 106 may be (2m−1)λ/4 (m is a naturalnumber of 1 or more and λ is a center wavelength of the light emittedfrom the light-emitting layer).

The light-emitting layer 104 may be formed in a single-layer structurewith a layer containing a light-emitting material, or may be formed in amultilayer structure including layers such as an electron-transportinglayer or a hole-transporting layer and a layer containing alight-emitting material. The layer containing the light-emittingmaterial may be a layer in which a light-emitting material to be aluminescence center is diffused or may be a layer consisting of alight-emitting material.

A plurality of layers formed with different materials is providedbetween the light-emission position and the second electrode 106. Inthis embodiment mode, the plurality of layers correspond to theelectron-generating layer 105 and the second hole-generating layer 103.A half of the layer containing the light-emitting material, having halfthe thickness thereof can be regarded as a layer positioned between thelight-emission position and the second electrode 106. In the case offorming the light-emitting layer with a plurality of layers, more layersformed with different materials may be included. In such a structure,the optical distance between the light-emission position and the secondelectrode 106 can be calculated by multiplying the thicknesses and therefractive indexes of the respective films and summing up the products.The total is set so as to be (2m−1)λ/4 (m is a natural number of 1 ormore and λ is a center wavelength of the light emitted from thelight-emitting layer). That is to say, the following formula (1) issatisfied. In the formula (1), the layer containing the light-emittingmaterial is assumed to be 1 and the second electrode 106 is assumed tobe j (j is an integer number of 4 or more), and the layers existingbetween the layer containing the light-emitting material and the secondelectrode 106 are denoted with numerals in order from the layercontaining the light-emitting material. Moreover, the refractive index nand the thickness d with a certain numeral given thereto indicate therefractive index and the thickness of the layer to which the samenumeral is given (that is, n₁ is the refractive index of the layercontaining the light-emitting material and d_(j) is the thickness of thesecond electrode).

$\begin{matrix}{{\sum\limits_{k = 2}^{j - 1}{n_{k}d_{k}}} \leq \frac{\left( {{2m} - 1} \right)\lambda}{4} \leq {{n_{1}d_{1}} + {\sum\limits_{k = 2}^{j - 1}{n_{k}d_{k}}}}} & \left( {{Formula}\mspace{14mu} 1} \right)\end{matrix}$

Here, it is necessary to control the film thickness in order to satisfythe formula (1). Since the layer mainly containing the organic compoundhas low electron mobility, the drive voltage increases when theelectron-generating layer 105 containing the electron-transportingmaterial in which electrons serve as carriers is thick. Consequently, inthis embodiment mode, the thickness of the second hole-generating layer103 in which the mobility is relatively high in the layer mainlycontaining the organic compound is controlled, whereby the formula (1)is satisfied without drastically increasing the drive voltage.

In the case of a light-emitting element in which light is extracted fromthe side of the second electrode 106, the light is reflected at thefirst electrode 101. Therefore, in order to increase the currentefficiency of the light-emitting element and to decrease the change ofthe spectrum shape which occurs depending on the angle of viewing thesurface from which the light is extracted, the optical distance(refractive index×physical distance) from the light-emission position tothe surface of the first electrode 101 may be set to (2m−1)λ/4 (m is anatural number of 1 or more and λ is a center wavelength of the lightemitted from the light emitting layer.)

The light-emitting layer 104 may be formed with a single-layer of thelayer containing the light-emitting material or in a multilayerstructure including the layer containing the light-emitting material andthe layers such as the electron-transporting layer or thehole-transporting layer. The layer containing the light-emittingmaterial may be a layer in which the light-emitting material to be thelight-emission center is diffused, or a layer consisting of thelight-emitting material. In any one of the above-mentioned structures,the layer containing the light-emitting material has a certain degree ofthickness and an infinite number of the luminescence centers exist;therefore, it is impossible to determine the exact position where thelight emission occurs. Accordingly, in this embodiment mode, a positionof a half of the film containing the light-emitting material, havinghalf the thickness thereof is regarded as the position where thelight-emission occurs.

One or a plurality of layers is provided between the position where thelight-emission occurs and the first electrode 101. In this embodimentmode, the layer corresponds to the first hole-generating layer 102.Further, a half of the layer containing the light-emitting material,having half the thickness thereof may be the layer located between theposition where the light-emission occurs and the first electrode 101.Moreover, more layers may be included in the case where thelight-emitting layer is formed with a plurality of layers. In such astructure, the optical distance from the light-emission position to thefirst electrode 101 can be calculated by multiplying the thicknesses andthe refractive indexes of the respective films and summing up theproducts. That is to say, the following formula (2) is satisfied. In theformula (2), the layer containing the light-emitting material is assumedto be 1 and the first electrode 101 is assumed to be j (j is an integernumber of 4 or more), and the layers existing between the layercontaining the light-emitting material and the first electrode 101 aredenoted with numerals in order from the layer containing thelight-emitting material. Moreover, the refractive index n and thethickness d with a certain numeral given thereto indicate the refractiveindex and the thickness of the layer to which the same numeral is given(that is, n₁ is the refractive index of the layer containing thelight-emitting material and d_(j) is the thickness of the firstelectrode).

$\begin{matrix}{{\sum\limits_{k = 2}^{j - 1}{n_{k}d_{k}}} \leq \frac{\left( {{2m} - 1} \right)\lambda}{4} \leq {{n_{1}d_{1}} + {\sum\limits_{k = 2}^{j - 1}{n_{k}d_{k}}}}} & \left( {{Formula}\mspace{14mu} 2} \right)\end{matrix}$

Here, it is necessary to control the film thickness in order to satisfythe formula (2). In this embodiment mode, the formula (2) can besatisfied without drastically increasing the drive voltage, bycontrolling the thickness of the first hole-generating layer 102 inwhich the mobility is relatively high in the layer mainly containing theorganic compound.

In the case of the structure in which light is extracted from both ofthe first electrode 101 and the second electrode 106, both of theformulas (1) and (2) may be satisfied.

With the structure of the light-emitting element shown in thisembodiment mode, it is possible to provide a light-emitting element inwhich the change of the light-emission spectrum which occurs dependingon the angle of viewing the surface from which the light is extracted isdecreased.

The present embodiment mode can be combined with Embodiment Mode 1.

Embodiment Mode 3

This embodiment mode will describe a display device according to thepresent invention shown in Embodiment Mode 1 or Embodiment Mode 2 whileshowing its manufacturing method with reference to FIGS. 5A to 6C.Although this embodiment mode shows an example of manufacturing anactive matrix display device, a light-emitting element of the presentinvention is also applicable for a passive matrix display device.

First, a first base insulating layer 51 a and a second base insulatinglayer 51 b are formed over a substrate 50, and then a semiconductorlayer is formed over the second base insulating layer 51 b (FIG. 5A).

As a material of the substrate 50, glass, quartz, plastic (such aspolyimide, acrylic, polyethylene terephthalate, polycarbonate,polyacrylate, or polyethersulfone), or the like can be used. Thesesubstrates may be used after being polished by CMP or the like asnecessary. In this embodiment mode, a glass substrate is used.

The first base insulating layer 51 a and the second base insulatinglayer 51 b are provided in order to prevent an element which adverselyaffects the characteristic of the semiconductor film such as alkalimetal or alkali-earth metal in the substrate 50 from diffusing into thesemiconductor layer. As the material of these base insulating layers,silicon oxide, silicon nitride, silicon oxide containing nitrogen,silicon nitride containing oxygen, or the like can be used. In thisembodiment mode, the first base insulating layer 51 a is formed withsilicon nitride, and the second base insulating layer 51 b is formedwith silicon oxide. Although the base insulating layer is formed in atwo-layer structure including the first base insulating layer 51 a andthe second base insulating layer 51 b in this embodiment mode, the baseinsulating layer may be formed in a single-layer structure or amultilayer structure including three or more layers. The base insulatinglayer is not necessary when the diffusion of the impurity from thesubstrate does not lead to a significant problem.

In this embodiment mode, the semiconductor layer formed subsequently isobtained by crystallizing an amorphous silicon film with a laser beam.The amorphous silicon film is formed to a thickness of 25 to 100 nm(preferably 30 to 60 nm) over the second base insulating layer 51 b by aknown method such as a sputtering method, a reduced-pressure CVD method,or a plasma CVD method. After that, heat treatment is conducted for onehour at 500° C. for dehydrogenation.

Next, the amorphous silicon film is crystallized with a laserirradiation apparatus to form a crystalline silicon film. In thisembodiment mode, an excimer laser is used at the laser crystallization.After the emitted laser beam is shaped into a linear beam spot using anoptical system, the amorphous silicon film is irradiated with the linearbeam spot. Thus, the crystalline silicon film is formed which is to beused as the semiconductor layer.

Alternatively, the amorphous silicon film can be crystallized by anothermethod such as a method in which the crystallization is conducted onlyby heat treatment or a method in which heat treatment is conducted usinga catalyst element for inducing the crystallization. As the element forinducing the crystallization, nickel, iron, palladium, tin, lead,cobalt, platinum, copper, gold, or the like is given. By using such anelement, the crystallization is conducted at lower temperature inshorter time than the crystallization only by the heat treatment;therefore, the damage to the glass substrate is suppressed. In the caseof crystallizing only by the heat treatment, a quartz substrate whichcan resist the high temperature is preferably used as the substrate 50.

Subsequently, a small amount of impurity elements are added to thesemiconductor layer as necessary in order to control the threshold,which is so-called channel doping. In order to obtain the requiredthreshold, an impurity showing N-type or P-type (such as phosphorus orboron) is added by an ion-doping method or the like.

After that, as shown in FIG. 5A, the semiconductor layer is patternedinto a predetermined shape so that an island-shaped semiconductor layer52 is obtained. The patterning is conducted by etching the semiconductorlayer using a mask. The mask is formed in such a way that a photo resistis applied to the semiconductor layer and the photo resist is exposedand baked so that a resist mask having a desired mask pattern is formedover the semiconductor layer.

Next, a gate insulating layer 53 is formed so as to cover thesemiconductor layer 52. The gate insulating layer 53 is formed to athickness of 40 to 150 nm with an insulating layer containing silicon bya plasma CVD method or a sputtering method. In this embodiment mode,silicon oxide is used.

Then, a gate electrode 54 is formed over the gate insulating layer 53.The gate electrode 54 may be formed with an element selected from thegroup consisting of tantalum, tungsten, titanium, molybdenum, aluminum,copper, chromium, and niobium, or may be formed with an alloy materialor a compound material which contains the above element as its maincomponent. Further, a semiconductor film typified by a poly-crystallinesilicon film doped with an impurity element such as phosphorus may beused. Ag—Pd—Cu alloy may also be used.

Although the gate electrode 54 is formed with a single layer in thisembodiment mode, the gate electrode 54 may have a multilayer structureincluding two or more layers of, for example, tungsten as a lower layerand molybdenum as an upper layer. Even in the case of forming the gateelectrode in the multilayer structure, the above-mentioned material ispreferably used. The combination of the above materials may also beselected appropriately. The gate electrode 54 is processed by etchingwith the use of a mask formed with a photo resist.

Subsequently, impurities are added to the semiconductor layer 52 so asto form a high concentration region using the gate electrode 54 as themask. According to this step, a thin film transistor 70 comprising thesemiconductor layer 52, the gate insulating layer 53, and the gateelectrode 54 is formed.

The manufacturing process of the thin film transistor is not limited inparticular, and may be modified appropriately so that a transistorhaving a desired structure can be manufactured.

Although this embodiment mode employs a top-gate thin film transistorusing the crystalline silicon film obtained by the lasercrystallization, a bottom-gate thin film transistor using an amorphoussemiconductor film can also be applied to a pixel portion. Not onlysilicon but also silicon germanium can be used for the amorphoussemiconductor. In the case of using silicon germanium, the concentrationof germanium preferably ranges from approximately 0.01 to 4.5 atomic %.

Moreover, a microcrystal semiconductor (semi-amorphous semiconductor)film which includes crystal grains each having a diameter of 0.5 to 20nm in the amorphous semiconductor may also be used. The microcrystalhaving the crystal grains with a diameter of 0.5 to 20 mm is alsoreferred to as a so-called microcrystal (μc).

Semi-amorphous silicon (also referred to as SAS), which belongs to thesemi-amorphous semiconductor, can be obtained by decomposing silicidegas according to glow discharging. As typical silicide gas, SiH₄ isgiven. Besides, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or the like can beused. By using the silicide gas after diluting the silicide gas withhydrogen or hydrogen and one or plural kinds of inert gas selected fromthe group consisting of helium, argon, krypton, and neon, SAS can beeasily formed. The silicide gas is preferably diluted with the dilutionratio of 1:10 to 1:1000. The reaction to form the film by thedecomposition according to glow discharging may be conducted at thepressure ranging from 0.1 to 133 Pa. The electric power for forming theglow discharging may be supplied at high frequency in the range of 1 to120 MHz, preferably 13 to 60 MHz. The substrate heat temperature ispreferably 300° C. or less, preferably in the range of 100 to 250° C.

The Raman spectrum of thus formed SAS shifts to the side of lowerwavenumber than 520 cm⁻¹. According to X-ray diffraction, diffractionpeaks of a silicon crystal lattice are observed at (111) and (220). As aterminating agent of a dangling bond, hydrogen or halogen is added by atleast 1 atomic % or more. As the impurity element in the film, theimpurity in the air such as oxygen, nitrogen, and carbon is desirably1×10²⁰ cm⁻¹ or less, and especially, the concentration of oxygen is5×10¹⁹/cm³ or less, preferably 1×10¹⁹/cm³ or less. The mobility of a TFTmanufactured with this film is μ=1 to 10 cm²/Vsec.

This SAS may be used after being crystallized further with a laser beam.

Subsequently, an insulating film (hydride film) 59 is formed withsilicon nitride so as to cover the gate electrode 54 and the gateinsulating layer 53. After forming the insulating film (hydride film)59, heat treatment for approximately 1 hour at 480° C. is conducted soas to activate the impurity element and to hydrogenate the semiconductorlayer 52.

Subsequently, a first interlayer insulating layer 60 is formed so as tocover the insulating film (hydride film) 59. As a material for formingthe first interlayer insulating layer 60, silicon oxide, acrylic,polyimide, siloxane, a low-k material, or the like is preferably used.In this embodiment mode, the first interlayer insulating layer is formedwith silicon oxide (FIG. 5B)

Next, contact holes that reach the semiconductor layer 52 are formed.The contact holes can be formed by etching with a resist mask until thesemiconductor layer 52 is exposed. Either wet etching or dry etching canbe applied. The etching may be conducted once or multiple timesdepending on the condition. When the etching is conducted multipletimes, both of the wet etching and the dry etching may be conducted(FIG. 5C).

Then, a conductive layer is formed so as to cover the contact holes andthe first interlayer insulating layer 60. A connection portion 61 a, awiring 61 b, and the like are formed by processing the conductive layerinto a desired shape. This wiring may be a single layer of aluminum,copper, or the like. In this embodiment mode, the wiring is formed in amultilayer structure of molybdenum/aluminum/molybdenum in order from thebottom. Alternatively, a structure of titanium/aluminum/titanium ortitanium/titanium nitride/aluminum/titanium is also applicable (FIG.5D).

A second interlayer insulating layer 63 is formed so as to cover theconnection portion 61 a, the wiring 61 b, and the first interlayerinsulating layer 60. As the material of the second interlayer insulatinglayer 63, an applied film having self-flattening properties such as afilm of acrylic, polyimide, siloxane, or the like is preferable. In thisembodiment mode, the second interlayer insulating layer 63 is formedwith siloxane (FIG. 5E).

Next, an insulating layer may be formed with silicon nitride over thesecond interlayer insulating layer 63. This is to prevent the secondinterlayer insulating layer 63 from being etched more than necessary ina later step of etching a pixel electrode. Therefore, the insulatinglayer is not necessary in particular when the difference of the etchingrate is large between the pixel electrode and the second interlayerinsulating layer. Next, a contact hole penetrating the second interlayerinsulating layer 63 to reach the connection portion 61 a is formed.

Next, after a light-transmitting conductive layer is formed so as tocover the contact hole and the second interlayer insulating layer 63 (orthe insulating layer), the light-transmitting conductive layer isprocessed to form the first electrode 101 of the thin filmlight-emitting element. Here, the first electrode (an electrode whichreceives electrons) 101 electrically contacts the connection portion 61a. As the material of the first electrode 101, it is preferable to usemetal, alloy, an electrically conductive compound, or mixture of theseeach of which has a high work function (work function of 4.0 eV ormore). For example, ITO (indium tin oxide), ITO containing silicon(ITSO), IZO (indium zinc oxide) in which zinc oxide (ZnO) is mixed by 2to 20% into indium oxide, zinc oxide, GZO (gallium zinc oxide) in whichgallium is contained in zinc oxide, gold (Au), platinum (Pt), nickel(Ni), tungsten (W), chromium (Cr), molybdenum (Mo), iron (Fe), cobalt(Co), copper (Cu), palladium (Pd), or metal nitride such as TiN can beused. In this embodiment mode, the first electrode 101 is formed withITSO (FIG. 6A).

Next, an insulating layer formed with an organic material or aninorganic material is formed so as to cover the second interlayerinsulating layer 63 (or the insulating layer) and the first electrode101. Subsequently, the insulating layer is processed so as to partiallyexpose the first electrode 101, thereby forming a partition wall 65. Asthe material of the partition wall 65, a photosensitive organic material(such as acrylic or polyimide) is preferable. Besides, anon-photosensitive organic material or inorganic material may also beused. Further, the partition wall 65 may be used as a black matrix bymaking the partition wall 65 black in such a way that a black pigment ordye such as titanium black or carbon nitride is diffused into thematerial of the partition wall 65 with the use of a diffuse material. Itis desirable that the partition wall 65 has a tapered shape in its endsurface toward the first electrode with its curvature changingcontinuously (FIG. 6B).

Next, a light-emitting stack 66 is formed so as to cover a part of thefirst electrode 101 that is exposed from the partition wall 65. In thisembodiment mode, the light-emitting stack 66 may be formed by anevaporating method or the like. The light-emitting stack 66 is formedwith the first hole-generating layer 102, the light-emitting layer 104,the electron-generating layer 105, and the second hole-generating layer103 stacked in order.

Further, the first hole generating layer 102 and the second holegenerating layer 103 are preferably formed with the same materials usinga hole transporting material and an electron accepting material whichcan receive electrons from the hole transporting material; P-typesemiconductor layer or a layer containing P-type semiconductor. As thehole-transporting material, for example, an aromatic amine compound(having a bond of a benzene ring with nitrogen), phthalocyanine(abbreviated to H₂Pc), or a phthalocyanine compound such as copperphthalocyanine (abbreviated to CuPc) or vanadyl phthalocyanine(abbreviated to VOPc) can be used. The aromatic amine compound is, forexample, 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl (abbreviatedto α-NPD), 4,4′-bis[N-(3-methylphenyl)-N-phenyl-amino]-biphenyl(abbreviated to TPD), 4,4′,4″-tris(N,N-diphenyl-amino)-triphenylamine(abbreviated to TDATA),4,4′,4′-tris[N-(3-methylphenyl)-N-phenyl-amino]-triphenylamine(abbreviated to MTDATA),4,4′-bis(N-(4-(N,N-di-m-tolylamino)phenyl)-N-phenylamino)biphenyl(abbreviated to DNTPD), or 1,3,5-tris[N,N-di(m-tolyl)amino]benzene(abbreviated to m-MTDAB). As the electron-accepting material which canreceive electrons from the hole-transporting material, for example,molybdenum oxide (MoOx), vanadium oxide,7,7,8,8,-tetracyanoquinodimethane (abbreviated to TCNQ),2,3-dicyanonaphtoquinone (abbreviated to DCNNQ),2,3,5,6-tetrafluoro-7,7,8,8,-tetracyanoquinodimethane (abbreviated toF4-TCNQ), or the like is used. The electron-accepting material isselected which can receive electrons in accordance with the combinationwith the hole-transporting material. Further, metal oxide such asmolybdenum oxide (MoOx), vanadium oxide, ruthenium oxide, cobalt oxide,nickel oxide, or copper oxide can be used as the P-type semiconductor.Note that the above materials are only example and the material can beappropriately selected by a practitioner. The mixture ratio of anelectron accepting material which can receive electrons from a holetransporting material to the hole transporting material may be 0.5 ormore, in molar ratio, preferably, 0.5 to 2. Further, the differencebetween the molar ratio of the electron accepting material to the holetransporting material in the second hole generating layer and the molarratio of the electron accepting material to the hole transportingmaterial in the first hole generating layer is preferably in the rangeof 80% of the molar ratio of the electron accepting material to the holetransporting material in the second hole generating layer and also inthe range of 80% of the molar ratio of the electron accepting materialto the hole transporting material in the first hole generating layer.More preferably, the difference between the molar ratio of the electronaccepting material to the hole transporting material in the second holegenerating layer and the molar ratio of the electron accepting materialto the hole transporting material in the first hole generating layer isin the range of 40% of the molar ratio of the electron acceptingmaterial to the hole transporting material in the second hole generatinglayer and also in the range of 40% of the molar ratio of the electronaccepting material to the hole transporting material in the first holegenerating layer. In the first hole generating layer and the second holegenerating layer in this embodiment, α-NPD is used as the electrontransporting material, and molybdenum oxide (MoO₃) is used as theelectron accepting material which can receive electrons from α-NPD.Deposition is performed by co-evaporation so as to satisfy α-NPD:MoO₃=4:1 (corresponds to 1 in molar ratio) by weight.

The layer containing a plurality of materials can be formed bydepositing the materials simultaneously. For example, the same methodsor different methods of co-evaporation by resistance heating,co-evaporation by electron beam deposition, co-evaporation by resistanceheating and electron beam deposition, deposition by resistance heatingand sputtering, deposition by electron beam deposition and sputtering,and the like may be combined to form the layer. In the above case, twomaterials are used; however, three or more materials may also be used toform the layer in the same manner.

The thickness of the first hole generating layer and the second holegenerating layer is to be 30 nm to 1 μm; further, the thickness of thesecond hole generating layer is not to be thicker or thinner than thatof the first hole generating layer by more than 50%. Meanwhile, thethickness of the first hole generating layer is not to be thicker orthinner than that of the second hole generating layer by more than 50%.More preferably, the thicknesses of the first hole generating layer andthe second hole generating layer are the same as in this embodimentmode. In this embodiment mode, the thickness of the first holegenerating layer is set to be 50 nm and the thickness of the second holegenerating layer is set to be 50 nm. The first hole generating layer 102and the second hole generating layer 103 are preferably formed with alayer containing both a hole transporting material and an electronaccepting material which can receive electrons from the holetransporting material. The difference between the molar ratio of theelectron accepting material to the hole transporting material in thesecond hole generating layer 103 and the molar ratio of the electronaccepting material to the hole transporting material in the first holegenerating layer 102 is preferably in the range of 80% of the molarratio of the electron accepting material to the hole transportingmaterial in the second hole generating layer 103 and also in the rangeof 80% of the molar ratio of the electron accepting material to the holetransporting material in the first hole generating layer 102. Morepreferably, the difference between the molar ratio of the electronaccepting material to the hole transporting material in the second holegenerating layer 103 and the molar ratio of the electron acceptingmaterial to the hole transporting material in the first hole generatinglayer 102 is in the range of 40% of the molar ratio of the electronaccepting material to the hole transporting material in the second holegenerating layer 103 and also in the range of 40% of the molar ratio ofthe electron accepting material to the hole transporting material in thefirst hole generating layer 102.

For the first hole generating layer 102 and the second hole generatinglayer 103, a first layer and a second layer, each of which is formedwith a layer containing the above described hole transporting materialand an inorganic material of zinc oxide, indium oxide, tin oxide,antimony oxide, indium nitride, tin nitride, antimony nitride, atungsten nitride, or molybdenum nitride. The thickness of the firstlayer and the second layer is preferably 30 nm to 1 μm. Further, thethicknesses of the first layer and the second layer are preferablyalmost the same, and the thickness of the second layer is not to bethicker or thinner than that of the first layer by more than 50%.Meanwhile, the thickness of the first layer is not to be thicker orthinner than that of the second layer by more than 50%. In other words,the thickness of the second layer is 50% to 150% of the thickness of thefirst layer, and the thickness of the first layer is 50% to 150% of thethickness of the second layer. Further, the first layer and the secondlayer are preferably formed with respective layers containing the samehole transporting material and the same inorganic material. Thedifference between the molar ratio of the electron accepting material tothe hole transporting material in the second layer and the molar ratioof the electron accepting material to the hole transporting material inthe first layer is preferably in the range of 80% of the molar ratio ofthe electron accepting material to the hole transporting material in thesecond layer and also in the range of 80% of the molar ratio of theelectron accepting material to the hole transporting material in thefirst layer. More preferably, the difference between the molar ratio ofthe electron accepting material to the hole transporting material in thesecond layer and the molar ratio of the electron accepting material tothe hole transporting material in the first layer is in the range of 40%of the molar ratio of the electron accepting material to the holetransporting material in the second layer and also in the range of 40%of the molar ratio of the electron accepting material to the holetransporting material in the first layer.

When the light-emitting layer 104 is formed with a layer in which alight-emitting material to be the light-emission center is diffused inthe layer containing the material having larger energy gap than thelight-emitting material, the following material can be used as thelight-emitting material to be the luminescence center;4-dicyanomethylene-2-methyl-6-[-2-(1,1,7,7-tetramethyl-9-julolidyl)ethenyl)-4H-pyran(abbreviation: DCJT);4-dicyanomethylene-2-t-butyl-6-[2-(1,1,7,7-tetramethyl-julolidine-9-yl)ethenyl]-4H-pyran;periflanthene;2,5-dicyano-1,4-bis[2-(10-methoxy-1,1,7,7-tetramethyl-julolidine-9-yl)ethenyl]benzene,N,N′-dimethylquinacridone (abbreviated to DMQd), coumarin 6, coumarin545T, tris (8-quinolinolato)aluminum (abbreviated to Alq₃),9,9′-bianthryl, 9,10-diphenylanthracene (abbreviated to DPA),9,10-bis(2-naphthyl)anthracene (abbreviated to DNA),2,5,8,11-tetra-t-butylperylene (abbreviated to TBP), or the like. As thematerial to be a base material in which the light-emitting material isdiffused, the following can be used; an anthracene derivative such as9,10-di(2-naphtyl)-2-tert-butylanthracene (abbreviated to t-BuDNA), acarbazole derivative such as 4,4′-bis(N-carbazolyl)biphenyl (abbreviatedto CBP), or a metal complex such as tris(8-quinolinolato)aluminum(abbreviated to Alq₃), tris(4-methyl-8-quinolinolato)aluminum(abbreviated to Almq₃), bis(10-hydroxybenzo[h]-quinolinato)beryllium(abbreviated to BeBq₂),bis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum (abbreviated toBAlq), bis[2-(2-hydroxyphenyl)pyridinato]zinc (abbreviated to Znpp₂), orbis[2-(2-hydroxyphenyl)benzoxazolate]zinc (abbreviated to ZnBOX). As thematerial which can constitute the light-emitting layer 104 singly,tris(8-quinolinolato)aluminum (abbreviated to Alq₃),9,10-bis(2-naphtyl)anthracene (abbreviated to DNA),bis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum (abbreviated toBAlq), or the like can be used to form the light emitting layer 104.

The light-emitting layer 104 may be formed either in a single-layerstructure or a multilayer structure. Moreover, a hole-transporting layermay be provided between the first hole-generating layer 102 and thelayer in which the light-emitting material is diffused in thelight-emitting layer 104 (or the layer containing the light-emittingmaterial). Further, an electron-transporting layer may be providedbetween the electron-generating layer 105 and the layer in which thelight-emitting material is diffused in the light-emitting layer 104 (orthe layer containing the light-emitting material). These layers are notnecessarily provided, and only one of the hole-transporting layer andthe electron-transporting layer may be provided. The materials of thehole-transporting layer and the electron-transporting layer conform tothose of the hole-transporting layer in the hole-generating layer andthe electron-transporting layer in the electron-generating layer;therefore, the description is omitted here. Refer to the description ofthose layers.

In this embodiment mode, the hole-transporting layer, the layer in whichthe light-emitting material is diffused, and the electron-transportinglayer are formed in order as the light-emitting layer 104 over thehole-generating layer 102. α-NPD is deposited to the thickness of 10 nmas the hole-transporting layer, Alq and coumarin 6 are deposited to thethickness of 35 nm at a weight ratio of 1:0.005 as the layer in whichthe light-emitting material is diffused, and Alq is deposited to thethickness of 10 nm as the electron-transporting layer.

As the electron-generating layer 105, a layer containing both of anelectron-transporting material and an electron donating material whichcan donate electrons to the electron-transporting material, an N-typesemiconductor layer, or a layer containing an N-type semiconductor canbe used. As the electron-transporting material, for example, thefollowing can be used; a metal complex having a quinoline skeleton or abenzoquinoline skeleton such as tris-(8-quinolinolato)aluminum(abbreviated to Alq₃), tris(4-methyl-8-quinolinolato)aluminum(abbreviated to Almq₃), bis(10-hydroxybenzo[h]-quinolinolato)beryllium(abbreviated to BeBq₂), orbis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum (abbreviated toBAlq). Besides, a metal complex having an oxazole or thiazole ligandsuch as bis[2-(2-hydroxyphenyl)benzoxazolate]zinc (abbreviated toZn(BOX)₂) or bis[2-(2-hydroxyphenyl)benzothiazolate]zinc (abbreviated toZn(BTZ)₂) can be used. In addition to the metal complex,2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviated toPBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazole-2-yl]benzene(abbreviated to OXD-7),3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole(abbreviated to TAZ),3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole(abbreviated to p-EtTAZ), bathophenanthroline (abbreviated to BPhen),bathocuproin (abbreviated to BCP), or the like can be used. As theelectron donating material which can donate electrons to theelectron-transporting material, for example, alkali metal such aslithium or cesium, alkali-earth metal such as magnesium or calcium, orrare-earth metal such as erbium or ytterbium can be used. The electrondonating material which can donate electrons is selected in accordancewith the combination with the electron-transporting material. Further,for example, zinc oxide, zinc sulfide, zinc selenide, titanium oxide, orthe like can be used the N-type semiconductor.

The mixture ratio between the electron-transporting material and theelectron-donating material which can supply electrons to theelectron-transporting material is approximately 1:0.5 to 1:2, preferably1:1, in molar ratio. The electron-transporting material is formed of Alqand the electron-donating material which can supply electrons to Alq isformed of lithium (Li) in the electron-generating layer in thisembodiment mode. Deposition is performed by the co-evaporating method soas to satisfy Alq:Li=1:0.01 in weight ratio. The film thickness is setto 10 nm.

A light-emitting element which emits light with a different emissionwavelength may be formed for each pixel to perform color display.Typically, light-emitting elements corresponding to the respectivecolors of R (red), G (green), and B (blue) are formed. Even in thiscase, the color purity can be increased and the pixel area can beprevented from mirroring (reflecting) by providing, at a side of thepixel from which the light is emitted, a filter (colored layer) whichtransmits light of the emission wavelength. By providing the filter(colored layer), a circular polarizing plate which has beenconventionally required can be omitted and the loss of the light emittedfrom the light-emitting element can be reduced. Moreover, the change ofthe color tone recognized when viewing the pixel area (display screen)obliquely can be reduced.

The light-emitting element can have a structure in which light of asingle color or a white color is emitted. In the case of using thelight-emitting element of the white color, a filter (color layer) totransmit the light of a particular wavelength is provided at a side ofthe pixel from which the light is emitted. Thus, the color display canbe conducted.

In order to form a light-emitting layer which emits white light, thewhite light can be obtained by stacking, for example, Alq₃, Alq₃ partlydoped with nile red, which is a pigment for red light emission, Alq₃,p-EtTAZ, and TPD (aromatic diamine) sequentially by vapor deposition.

Moreover, the light-emitting layer may be formed using a triplet-excitedlight-emitting material containing a metal complex and the like otherthan a singlet-excited light-emitting material. For example, among a redlight-emitting pixel, a green light-emitting pixel, and a bluelight-emitting pixel; the red light-emitting pixel, which has arelatively short half-life period, is formed with the triplet excitedlight-emitting material, and the others are formed with thesinglet-excited light-emitting material. Since the triplet-excited lightemitting material has high emission efficiency, less power is consumedto obtain the same luminance. Accordingly, in the case of applying thetriplet-excited light-emitting material to the red pixel, thereliability can be improved because the amount of current flowing intothe light-emitting element is small. In order to reduce lower powerconsumption, the red light-emitting pixel and the green light-emittingpixel may be formed with a triplet-excited light-emitting material, anda blue light-emitting pixel may be formed with the singlet-excitedlight-emitting material. By forming the green light-emitting element, towhich visual sensitivity is high, also with the triplet-excitedlight-emitting material, power consumption can be reduced.

As an example of the triplet-excited light-emitting material, thefollowing can be employed; a material using a metal complex as a dopant,such as a metal complex containing platinum, which is one of thirdtransition elements, as metal center or a metal complex containingiridium as metal center. The triplet-excited light-emitting material isnot limited to these compounds, and another compound which has the abovestructure and contain an element belonging to any one of groups 8 to 10in the periodic table as metal center can be used.

The light-emitting element formed with the above material emits light bybeing forward biased. A pixel of a display device formed using thelight-emitting element can be driven by a simple matrix method or anactive matrix method. In any way, the respective pixels emit light bybeing applied with forward bias at a particular timing and do not emitlight for a certain period. The reliability of the light-emittingelement can be increased by applying a reverse bias in this non-emissionperiod. The light-emitting element has deterioration modes in whichlight-emission intensity decreases under a certain drive condition orthe luminance seems to decrease because a non-emission region expandswithin the pixel. However, when alternate driving is conducted byapplying bias in the forward and reverse directions, the progress of thedeterioration can be slowed down and the reliability of thelight-emitting device can be improved.

Subsequently, the second electrode 106 is formed so as to cover thelight-emitting stack 66. Accordingly, a light-emitting element 93including the first electrode 101, the light-emitting stack 66, and thesecond electrode 106 can be manufactured. Note that, in this embodimentmode, when voltage is applied so as to make a light emitting elementemit light, the electrode applied with higher potential is the firstelectrode, and the electrode applied with lower potential is the secondelectrode. As the second electrode material used for forming the secondelectrode 106, it is preferable to use metal, an alloy, an electricallyconductive compound, a mixture thereof, or the like each of which haslow work function (work function of 3.8 eV or less). As a specificexample of the second electrode material, the following can be used; anelement belonging to group 1 or 2 in the periodic table, that is, analkali metal such as Li or Cs, an alkaline-earth metal such as Mg, Ca orSr, an alloy containing those elements such as Mg:Ag or Al:Li, or acompound containing those elements such as LiF, CsF, or CaF₂. Inaddition, the second electrode can also be formed with a transitionmetal containing a rare-earth metal. Further, a multilayer containingthe above element and another metal (including an alloy) such as Al, Ag,or ITO can be used. In this embodiment mode, the second electrode isformed of aluminum.

In the light-emitting element having the above structure, the drivevoltage is low and increase in the drive voltage over time is small.

Further, the stress to the light emitting layer can be alleviated, sothat the reliability of the light emitting element can be improved.

The electrode in electrical contact with the connection portion 61 a isthe first electrode 101 in this embodiment mode. However, the electrodein electrical contact with the connection portion 61 a may be the secondelectrode 106. In this case, the light-emitting stack 66 may be formedby stacking the second hole-generating layer 103, theelectron-generating layer 105, the light-emitting layer 104, and thefirst hole-generating layer 102 in order, and the first electrode 101may be formed over the light-emitting stack 66.

After that, a silicon oxide film containing nitrogen is formed as asecond passivation film by plasma CVD. In the case of using the siliconoxide film containing nitrogen, a silicon oxynitride film manufacturedfrom SiH₄, N₂O, and NH₃ by plasma CVD, a silicon oxynitride filmmanufactured from SiH₄ and N₂O by plasma CVD, or a silicon oxynitridefilm manufactured from a gas in which SiH₄ and N₂O are diluted with Arby plasma CVD is preferably formed.

As a first passivation film, a silicon oxynitride hydride filmmanufactured with SiH₄, N₂O, and H₂ is also applicable. The structure ofthe first passivation film is not limited to a single-layer structure,and the first passivation film may be formed in a single-layer structureor a multilayer structure of another insulating layer containingsilicon. A multilayer film of a carbon nitride film and a siliconnitride film, a multilayer film of styrene polymer, a silicon nitridefilm, or a diamond-like carbon film may be formed instead of a siliconoxide film containing nitrogen.

Subsequently, in order to protect the light emitting element from adeterioration-promoting material such as moisture, the display portionis sealed. In the case of using a counter substrate for the sealing, thecounter substrate and an element substrate are pasted together by aninsulating sealing material so as to expose an external connectionportion. The space between the counter substrate and the elementsubstrate may be filled with an inert gas such as dry nitrogen, or thesealing material may be applied to the whole surface of the pixel areafor pasting the counter substrate. It is preferable to use anultraviolet curable resin or the like as the sealing material. A dryingagent or particles for keeping the gap between the substrates uniformmay be mixed into the sealing material. Subsequently, a flexible wiringsubstrate is pasted on the external connection portion, therebycompleting the display device.

An example of the structure of the thus manufactured display device willbe described with reference to FIGS. 7A and 7B. Although the shapes aredifferent, the same parts having the same function are denoted with thesame reference numerals and the description to such parts may beomitted. In this embodiment mode, the thin film transistor 70 having anLDD structure connects to the light-emitting element 93 via theconnection portion 61 a.

In FIG. 7A, the first electrode 101 is formed with a light-transmittingconductive film and has a structure in which light emitted from thelight-emitting stack 66 is extracted to the side of the substrate 50.Reference numeral 94 denotes a counter substrate, which is to be fixedto the substrate 50 with a sealing material or the like after formingthe light-emitting element 93. By filling the space between the countersubstrate 94 and the element with a light-transmitting resin 88 or thelike and sealing the space, it is possible to prevent the light-emittingelement 93 from deteriorating due to the moisture. Further, the resin 88desirably has hygroscopicity. In addition, it is more desirable that adrying agent 89 having high light-transmitting properties is diffused inthe resin 88, so that the effect of moisture can be suppressed further.

In FIG. 7B, either the first electrode 101 or the second electrode 106is formed with a light-transmitting conductive film and has a structurein which light can be extracted toward both of the substrate 50 and thecounter substrate 94. With this structure, it is possible to prevent thescreen from becoming transparent by respectively providing polarizingplates 90 outside the substrate 50 and the counter substrate 94, wherebyincreasing the visibility. A protective film 91 is preferably providedoutside the polarizing plate 90.

Either an analog video signal or a digital video signal may be used inthe display device having a display function according to the presentinvention. The digital video signal includes a video signal usingvoltage and a video signal using current. When the light-emittingelement emits light, the video signal inputted into the pixel uses aconstant voltage or a constant current. When the video signal uses aconstant voltage, the voltage applied to the light-emitting element orthe current flowing in the light-emitting element is constant. On theother hand, when the video signal uses the constant current, the voltageapplied to the light-emitting element or the current flowing in thelight-emitting element is constant. The light-emitting element to whichthe constant voltage is applied is driven by the constant voltage, andthe light-emitting element in which the constant current flows is drivenby the constant current. A constant current flows in the light-emittingelement driven by the constant current without being affected by thechange of the resistance of the light-emitting element. Either one ofconstant voltage drive or constant current drive and a video signal ofeither voltage or current can be used for a light emitting device andthe driving method thereof according to the present invention.

In the display device according to the present invention manufactured bythe method in this embodiment mode, the drive voltage is low and theincrease in the drive voltage over time is small. Further, the displaydevice is highly reliable.

Embodiment Mode 4

This embodiment mode will describe an appearance of a panel of alight-emitting device corresponding to one aspect of the presentinvention with reference to FIGS. 8A and 8B. FIG. 8A is a top view of apanel in which a transistor and a light-emitting element formed over asubstrate are sealed with a sealing material formed between thesubstrate and a counter substrate 4006. FIG. 8B is a cross-sectionalview corresponds to FIG. 8A. The light-emitting element mounted in thispanel has a structure in which respective layers in contact with a pairof electrodes consisting of a first electrode and a second electrode arehole-generating layers, so that a light-emitting layer is sandwichedbetween the hole-generating layers. Moreover, in the light-emittingelement, an electron-generating layer is provided between thehole-generating layer on a second electrode side and the light-emittinglayer. In this embodiment mode, when voltage is applied so as to make alight emitting element emit light, the electrode applied with higherpotential is the first electrode, and the electrode applied with lowerpotential is the second electrode. The hole generating layers are formedof the same material. The thickness of the hole generating layer formedon the counter substrate side is not thicker or thinner than that of thehole generating layer formed on the substrate side by more than 50%.Further, the thickness of the hole generating layer formed on thesubstrate side is not thicker or thinner than that of the holegenerating layer formed on the counter substrate side by more than 50%.The thickness of the hole generating layer formed on the countersubstrate side and the hole generating layer formed on the substrateside is to be 30 nm to 1 μm.

A sealing material 4005 is provided so as to surround a pixel area 4002,a signal line driver circuit 4003, and a scan line driver circuit 4004which are provided over a substrate 4001. In addition, the countersubstrate 4006 is provided over the pixel area 4002, the signal linedriver circuit 4003, and the scan line driver circuit 4004. Thus, thepixel area 4002, the signal line driver circuit 4003, and the scan linedriver circuit 4004 together with the filler 4007 are sealed with thesubstrate 4001, the sealing material 4005, and the counter substrate4006.

The pixel area 4002, the signal line driver circuit 4003, and the scanline driver circuit 4004 provided over the substrate 4001 have aplurality of thin film transistors. FIG. 8B shows a thin film transistor4008 included in the signal line driver circuit 4003 and a thin filmtransistor 4010 included in the pixel area 4002.

The light-emitting element 4011 is connected electrically to the thinfilm transistor 4010.

Further, a lead wiring 4014 corresponds to a wiring for supplying asignal or a power source voltage to the pixel area 4002, the signal linedriver circuit 4003, and the scan line driver circuit 4004. The leadwiring 4014 is connected to a connection terminal 4016 via a lead wiring4015 a and a lead wiring 4015 b. The connection terminal 4016 iselectrically connected to a terminal of a flexible printed circuit (FPC)4018 via an anisotropic conductive film 4019.

As the filler 4007, in addition to inert gas such as nitrogen or argon,an ultraviolet curable resin or a thermosetting resin can be used. Forexample, polyvinyl chloride, acrylic, polyimide, an epoxy resin, asilicon resin, polyvinyl butyral, or ethylene vinylene acetate can beused.

It is to be noted that the display device according to the presentinvention includes in its category the panel in which the pixel areahaving the light-emitting element is formed and a module in which an ICis mounted on the panel.

In the panel and the module having the structure shown in thisembodiment mode, the drive voltage is low and increase in the drivevoltage over time is small. Further, the reliability of the panel andthe module is high.

Embodiment Mode 5

As an electronic appliance according to the present invention to which amodule, for example, the module which has been exemplified in EmbodimentMode 4, is mounted, the following is given; a camera such as a videocamera or a digital camera, a goggle type display (head mount display),a navigation system, a sound reproduction device (car audio component orthe like), a computer, a game machine, a mobile information terminal (amobile computer, a mobile telephone, a mobile game machine, anelectronic book, or the like), an image reproduction device equippedwith a recording medium (specifically a device which reproduces therecording medium such as a digital versatile disc (DVD) and which isequipped with a display for displaying the image), or the like. FIGS. 9Ato 9E show specific examples of those electronic appliances.

FIG. 9A shows a light-emitting display device, which corresponds to, forexample, a television receiving device or a monitor of a personalcomputer. The light-emitting display device according to the presentinvention includes a case 2001, a display portion 2003, speaker portions2004, and the like. In the light-emitting display device according tothe present invention, the drive voltage of the display portion 2003 islow and increase in the drive voltage of the display portion 2003 overtime is small. Further, the reliability of the display portion 2003 ishigh. In the pixel area, a polarizing plate or a circular polarizingplate is preferably provided in the pixel area to improve the contrast.For example, films are preferably provided in the order of a quarterwave-plate, a half wave-plate, and a polarizing plate on a sealingsubstrate. Further, an anti-reflection film may be provided over thepolarizing plate.

FIG. 9B shows a mobile phone including a main body 2101, a case 2102, adisplay portion 2103, an audio input portion 2104, an audio outputportion 2105, operation keys 2106, an antenna 2108, and the like. In thedisplay portion 2103 of the mobile phone according to the presentinvention, the drive voltage is low and increase in the drive voltageover time is small. Further, the reliability of the display portion 2103is high.

FIG. 9C shows a computer including a main body 2201, a case 2202, adisplay portion 2203, a keyboard 2204, an external connection portion2205, a pointing mouse 2206, and the like. In the display portion 2203of the computer according to the present invention, the drive voltage islow and increase in the drive voltage over time is small. Further, thereliability of the display portion 2203 is high. Although FIG. 9C showsa laptop computer, the present invention is also applicable to a desktopcomputer in which a hard disk is integral with a display portion.

FIG. 9D shows a mobile computer including a main body 2301, a displayportion 2302, a switch 2303, operation keys 2304, an infrared port 2305,and the like. In the display portion 2302 of the mobile computeraccording to the present invention, the drive voltage is low and theincrease in the drive voltage over time is small. Further, thereliability of the display portion 2302 is high.

FIG. 9E shows a mobile game machine including a case 2401, a displayportion 2402, speaker portions 2403, operation keys 2404, an recordingmedium insert portion 2405, and the like. In the display portion 2402 ofthe mobile game machine according to the present invention, the drivevoltage is low and the increase in the drive voltage over time is small.Further, the reliability of the display portion 2402 is high.

As thus described, the present invention can be widely applied, and canbe used in electronic appliances of every field.

Embodiment Mode 6

FIGS. 10A to 10C show examples of bottom emission, dual emission, andtop emission, respectively. The structure whose manufacturing processhas been described in Embodiment Mode 3 corresponds to the structure inFIG. 10C. FIGS. 10A and 10B show the structures in which a firstinterlayer insulating layer 900 in FIG. 10C is formed of a materialhaving self-flattening properties and a wiring to be connected to a thinfilm transistor 901 and the first electrode 101 of the light-emittingelement are formed over the same interlayer insulating layer. In FIG.10A, the first electrode 101 in the light-emitting element is formedwith a light-transmitting material, and light is emitted toward a lowerpart of the light-emitting device, which is called a bottom-emissionstructure. In FIG. 10B, the second electrode 106 is formed with alight-transmitting material such as ITO, ITSO, or IZO, and light isextracted from both sides, which is called a dual-emission structure.When a film is formed with aluminum or silver thickly, the film does nottransmit light; however, the film transmits light when the film isformed thinly. Therefore, by forming the second electrode 106 withaluminum or silver in such a thickness that light can pass therethrough,dual emission can be achieved. Note that, in this embodiment mode, whenvoltage is applied so as to make a light emitting element emit light,the electrode applied with higher potential is the first electrode, andthe electrode applied with lower potential is the second electrode.Further, FIG. 10C shows a top-emission structure in which the secondelectrode 106 in the light-emitting element is formed with alight-transmitting material, and light is emitted toward an upper partof the light-emitting device.

Embodiment Mode 7

This embodiment mode will describe a pixel circuit and a protectivecircuit in the panel and the module shown in Embodiment Mode 4, andtheir operations. FIGS. 5A to 6C show cross sectional views of a driverTFT 1403 and a light-emitting element 1405 in FIGS. 11A to 11F.

A pixel shown in FIG. 11A includes a signal line 1410 and power sourcelines 1411 and 1412 in a column direction and a scan line 1414 in a rowdirection. The pixel further includes a switching TFT 1401, the driverTFT 1403, a current control TFT 1404, a capacitor 1402, and thelight-emitting element 1405.

A pixel shown in FIG. 11C has the same structure as one in FIG. 11Aexcept for that a gate electrode of the driver TFT 1403 is connected tothe power source line 1412 provided in the row direction. In otherwords, the pixels shown in FIGS. 11A and 11C have the same equivalentcircuit diagram. However, in the case of arranging the power source line1412 in the column direction (FIG. 11A) and in the case of arranging thepower source line 1412 in the row direction (FIG. 11C), each powersource line is formed with a conductive film in different layers. Here,attention is paid to a wiring connected to the gate electrode of thedriver TFT 1403, and the structure is shown separately in FIGS. 11A and11C in order to show that these wirings are manufactured with differentlayers.

As a feature of the pixels shown in FIGS. 11A and 11C, the driver TFT1403 and the current control TFT 1404 are connected serially within thepixel, and it is preferable to set the channel length L (1403) and thechannel width W (1403) of the driver TFT 1403, and the channel length L(1404) and the channel width W (1404) of the current control TFT 1404 soas to satisfy L (1403)/W (1403):L (1404)/W (1404)=5 to 6000:1.

The driver TFT 1403 operates in a saturation region and serves tocontrol the current value of the current flowing into the light-emittingelement 1405. The current control TFT 1404 operates in a linear regionand serves to control the current supply to the light-emitting element1405. Both TFTs preferably have the same conductivity type in themanufacturing step, and the TFTs are n-channel type TFTs in thisembodiment mode. The driver TFT 1403 may be either an enhancement typeor a depletion type. Since the current control TFT 1404 operates in thelinear region according to the present invention having the abovestructure, slight fluctuation of Vgs of the current control TFT 1404does not affect the current value of the light-emitting element 1405.That is to say, the current value of the light-emitting element 1405 canbe determined by the driver TFT 1403 operating in the saturation region.With the above structure, the unevenness of the luminance of thelight-emitting element due to the variation of the characteristic of theTFT can be improved, thereby providing a display device in which theimage quality is improved.

In the pixels shown in FIGS. 11A to 11D, the switching TFT 1401 is tocontrol the input of the video signal to the pixel, and the video signalis inputted into the pixel when the switching TFT 1401 is turned on.Then, the voltage of the video signal is held in the capacitor 1402.Although FIGS. 11A and 11C show the structure in which the capacitor1402 is provided, the present invention is not limited thereto. When thegate capacitance and the like can serve as a capacitor holding the videosignal, the capacitor 1402 is not necessarily provided.

A pixel shown in FIG. 11B has the same pixel structure as that in FIG.11A except for that a TFT 1406 and a scan line 1415 are added. In thesame way, a pixel shown in FIG. 11D has the same pixel structure as thatin FIG. 11C expect that the TFT 1406 and the scan line 1415 are added.

On and off of the TFT 1406 is controlled by the additionally providedscan line 1415. When the TFT 1406 is turned on, the charge held in thecapacitor 1402 is discharged, thereby turning off the current controlTFT 1404. In other words, by the provision of the TFT 1406, a state canbe produced compellingly in which the current is not flowed into thelight-emitting element 1405. For this reason, the TFT 1406 can bereferred to as an eraser TFT. Consequently, in the structures shown inFIGS. 11B and 11D, a lighting period can be started at the same time asor just after the start of a writing period before the writing of thesignal into all the pixels; therefore the duty ratio can be increased.

In a pixel shown in FIG. 11E, the signal line 1410 and the power sourceline 1411 are arranged in the column direction, and the scan line 1414is arranged in the row direction. Further, the pixel includes theswitching TFT 1401, the driver TFT 1403, the capacitor 1402, and thelight-emitting element 1405. A pixel shown in FIG. 11F has the samepixel structure as that shown in FIG. 11E except for that the TFT 1406and a scan line 1415 are added. In the structure shown in FIG. 11F, theduty ratio can also be increased by the provision of the TFT 1406.

As thus described, various pixel circuits can be employed. Inparticular, in the case of forming a thin film transistor from anamorphous semiconductor film, the semiconductor film for the driver TFT1403 is preferably large. In the case where the semiconductor is large,in the above pixel circuit, a top emission type is preferable in whichlight from an electroluminescent layer is emitted from the sealingsubstrate side.

Such an active matrix light-emitting device can be driven at low voltagewhen the pixel density increases, because the TFTs are provided inrespective pixels. Therefore, it is considered that the active matrixlight-emitting device is advantageous.

Although this embodiment mode will describe the active matrixlight-emitting device in which the respective TFTs are provided inrespective pixels, a passive matrix light-emitting device can also beformed in which TFTs are provided in each column. Since the TFTs are notprovided in respective pixels in the passive matrix light-emittingdevice, high aperture ratio can be obtained. In the case of alight-emitting device in which light is emitted to both sides of theelectroluminescent layer, the transmissivity of the passive matrixdisplay device is increased.

In the display device further comprising such pixel circuits accordingto the present invention, the drive voltage is low and increase in thedrive voltage over time is small. Moreover, the display device has therespective characteristics.

Subsequently, a case will be described in which a diode is provided as aprotective circuit on the scan line and the signal line with the use ofan equivalent circuit shown in FIG. 11E.

In FIG. 12, the switching TFT 1401, the driving TFT 1403, the capacitor1402, and the light-emitting element 1405 are provided in a pixel area1500. Diodes 1561 and 1562 are provided to the signal line 1410. In thesimilar way to the switching TFT 1401 and the driving TFT 1403, thediodes 1561 and 1562 are manufactured based on the above embodimentmodes, and have a gate electrode, a semiconductor layer, a sourceelectrode, a drain electrode, and the like. The diodes 1561 and 1562 areoperated as diodes by connecting the gate electrode with the drainelectrode or the source electrode.

Wirings 1554 and 1555 connecting to the diodes are formed with the samelayer as the gate electrode. Therefore, in order to connect the wirings1554 and 1555 with the source electrode or the drain electrode of thediode, it is necessary to form a contact hole in the gate insulatinglayer.

A diode provided on the scan line 1414 has the similar structure.

As thus described, according to the present invention, a protectivediode to be provided on an input stage can be manufacturedsimultaneously. The position at which the protective diode is formed isnot limited to this, and the diode may also be provided between thedriver circuit and the pixel.

In the display device having such protective circuits according to thepresent invention, the increase in the drive voltage over time is smallbesides the drive voltage is low and the reliability of the displaydevice can be improved.

Embodiment 1

This embodiment shows measurement data of a light-emitting elementaccording to the present invention.

First, a manufacturing method of a light-emitting element in thisembodiment is described. The light-emitting element in this embodimentconforms to the structure of the light-emitting element shown inEmbodiment Mode 1. In this embodiment, a glass substrate is used as theinsulator 100. ITO containing silicon is formed over the glass substrateby a sputtering method, thereby forming the first electrode 101. Thethickness of the first electrode 101 is set to 110 nm.

Subsequently, the first hole-generating layer 102 is formed withmolybdenum oxide (MoOx) and α-NPD by co-evaporating molybdenum oxide(VI) and α-NPD over the first electrode 101. Here, the thickness of thefirst hole-generating layer 102 is set to 50 nm. Note that the molarratio of α-NPD and a molybdenum oxide (MoOx) is to be 1:1.

Next, the light-emitting layer 104 is formed over the firsthole-generating layer 102. The light-emitting layer 104 is formed in athree-layer structure in which a hole-transporting layer, a layer wherea light-emitting material is diffused, and an electron-transportinglayer are stacked in order from the side of the first hole-generatinglayer 102. The hole-transporting layer is formed with α-NPD in 10 nmthick by a vacuum evaporating method. The layer in which thelight-emitting material is diffused is formed with Alq₃ and coumarin 6in 35 nm thick by a co-evaporating method. The electron-transportinglayer is formed with only Alq₃ in 10 nm thick by a vacuum evaporatingmethod. The layer in which the light-emitting material is diffused isadjusted so that the proportion between Alq₃ and coumarin 6 is 1:0.005in weight ratio.

Subsequently, the electron-generating layer 105 is formed with Alq₃ andlithium in 10 nm thick by co-evaporating Alq₃ and lithium over thelight-emitting layer 104. Alq₃ and lithium are adjusted so that theweight ratio between Alq₃ and lithium is 1:0.01.

Next, the second hole-generating layer 103 is formed with molybdenumoxide (MoOx) and α-NPD by co-evaporating molybdenum oxide (VI) and α-NPDover the electron-generating layer 105. Here, the thickness of the firsthole-generating layer 102 is set to 20 nm. The molar ratio between α-NPDand a molybdenum oxide (MoOx) is 1:1.

The second electrode 106 is formed with aluminum in 100 nm thick overthe second hole-generating layer 105.

When voltage is applied to the light-emitting element having the abovestructure according to the present invention, holes are injected fromthe second hole-generating layer 103 to the second electrode. Moreover,electrons are injected from the electron-generating layer 105 to thelight-emitting layer 104. Further, holes are injected from the firsthole-generating layer 102 to the light-emitting layer 104. Then, theinjected holes and electrons are recombined in the light-emitting layer,thereby providing light from coumarin 6.

FIG. 13 shows the voltage-luminance characteristic of the thusmanufactured light-emitting element of this embodiment, while FIG. 14shows the voltage-current characteristic thereof. In FIG. 13, thehorizontal axis shows the voltage (V), and the vertical axis shows theluminance (cd/m²). In FIG. 14, the horizontal axis shows the voltage(V), and the vertical axis shows the current (mA).

Thus, the light-emitting element in this embodiment exhibits superiorcharacteristic.

Note that, in this embodiment mode, when voltage is applied so as tomake a light emitting element emit light, the electrode applied withhigher potential is the first electrode, and the electrode applied withlower potential is the second electrode. A light emitting element ofthis embodiment has a structure in which the light emitting layer 104 isprovided between the first hole generating layer 102 in contact with thefirst electrode and the second hole generating layer 103 in contact withthe second electrode. The thickness of the second hole generating layer103 is not thicker or thinner than that of the first hole generatinglayer 102 by more than 50%. Further, the thickness of the first holegenerating layer 102 is not thicker or thinner than that of the secondhole generating layer 103 by more than 50%. Each of the hole generatinglayers has a thickness in the range of 30 nm to 1 μm. Further, thedifference between the molar ratio of the molybdenum oxide (MoOx) toα-NPD forming the second hole generating layer 103 and the molar ratioof the molybdenum oxide (MoOx) to α-NPD forming the first holegenerating layer 102 is in the range of 80% of the molar ratio of themolybdenum oxide (MoOx) to α-NPD forming the second hole generatinglayer 103 and also in the range of 80% of the molybdenum oxide (MoOx) toα-NPD forming the first hole generating layer 102.

Accordingly, the stress to the light emitting layer can be alleviated,so that the reliability of the light emitting element can be improved.

Embodiment 2

This embodiment describes a manufacturing method of four light-emittingelements having different mixture proportions between ahole-transporting material and an electron-accepting material whichshows electron-accepting properties to the hole-transporting material ina hole-generating layer. The four light-emitting elements are denoted bya light-emitting element (1), a light-emitting element (2), alight-emitting element (3), and a light-emitting element (4). Moreover,this embodiment describes the characteristics of these elements.

First, the manufacturing method of the light-emitting element in thisembodiment is described. In this embodiment, the light-emitting elementconforms to the structure of the light-emitting element shown inEmbodiment Mode 1. In this embodiment, a glass substrate is used as theinsulator 100. ITO containing silicon is formed over the glass substrateby a sputtering method, thereby forming the first electrode 101. Thethickness of the first electrode 101 is set to 110 nm.

Subsequently, the first hole-generating layer 102 is formed withmolybdenum oxide (MoOx) over the first electrode 101 by a vacuumevaporating method. Here, the thickness of the first hole-generatinglayer 102 is set to 5 nm.

Next, the light-emitting layer 104 is formed over the firsthole-generating layer 102. The light-emitting layer 104 is formed in athree-layer structure in which a hole-transporting layer, a layer wherea light-emitting material is diffused, and an electron-transportinglayer are stacked in order from the side of the first hole-generatinglayer 102. The hole-transporting layer is formed with α-NPD in 55 nmthick by a vacuum evaporating method. The layer in which thelight-emitting material is diffused is formed with Alq₃ and coumarin 6in 35 nm thick by a co-evaporating method. The electron-transportinglayer is formed with only Alq₃ in 10 nm thick by a vacuum evaporatingmethod. The layer in which the light-emitting material is diffused isadjusted so that the proportion between Alq₃ and coumarin 6 is 1:0.005in weight ratio.

Subsequently, the electron-generating layer 105 is formed with Alq₃ andlithium in 10 nm thick by co-evaporating Alq₃ and lithium over thelight-emitting layer 104. Alq₃ and lithium are adjusted so that theweight ratio between Alq₃ and lithium is 1:0.01.

Next, the second hole-generating layer 103 is formed with molybdenumoxide (MoOx) and α-NPD by co-evaporating molybdenum oxide (VI) and α-NPDover the electron-generating layer 105. Here, the light-emitting element(1) is adjusted so that the molar ratio of α-NPD to a molybdenum oxide(MoOx) is 0.5 (=molybdenum oxide (MoOx)/α-NPD). The light-emittingelement (2) is adjusted so that the molar ratio of α-NPD to molybdenumoxide (MoOx) is 1.0 (=molybdenum oxide (MoOx)/α-NPD). The light-emittingelement (3) is adjusted so that the molar ratio of α-NPD to molybdenumoxide (MoOx) is 1.5 (=molybdenum oxide (MoOx)/α-NPD). The light-emittingelement (4) is adjusted so that the molar ratio of α-NPD to molybdenumoxide (MoOx) is 2.0 (=molybdenum oxide (MoOx)/α-NPD). The thickness ofthe first hole-generating layer 102 is set to 20 nm.

The second electrode 106 is formed with aluminum in 100 nm thick overthe second hole-generating layer 103. Note that, in this embodimentmode, when voltage is applied so as to make a light emitting elementemit light, the electrode applied with higher potential is the firstelectrode, and the electrode applied with lower potential is the secondelectrode.

When voltage is applied to the light-emitting element having the abovestructure according to the present invention, holes are injected fromthe second hole-generating layer 103 to the second electrode. Moreover,electrons are injected from the electron-generating layer 105 to thelight-emitting layer 104. Further, holes are injected from the firsthole-generating layer 102 to the light-emitting layer 104. Then, theinjected holes and electrons are recombined in the light-emitting layer,thereby providing light from coumarin 6.

FIG. 15 shows the voltage-luminance characteristic of the light-emittingelement in the present embodiment. FIG. 16 shows the currentdensity-luminance characteristic thereof, and FIG. 17 shows thevoltage-current characteristic thereof. In FIG. 15, the horizontal axisshows the voltage (V) and the vertical axis shows the luminance (cd/m²).In FIG. 16, the horizontal axis shows the current density (mA/cm²) andthe vertical axis shows the luminance (cd/m²). In FIG. 17, thehorizontal axis shows the voltage (V) and the vertical axis shows thecurrent (mA). In FIGS. 15 to 17, ▴ shows the characteristic of thelight-emitting element (1), ● shows the characteristic of thelight-emitting element (2), ◯ shows the characteristic of thelight-emitting element (3), and ▪ shows the characteristic of thelight-emitting element (4).

It is to be understood from FIGS. 15 to 17 that all of thelight-emitting elements operate well. In the light-emitting elements (2)to (4) in which the molar ratio of α-NPD to molybdenum oxide (MoOx) inthe hole generating layer (=α-NPD/molybdenum oxide (MoOx)) ranges from 1to 2, high luminance is obtained by applying any voltage and highcurrent value is also obtained. Thus, the light-emitting element can beobtained which operates at lower drive voltage by adjusting the molarratio of α-NPD to molybdenum oxide (=α-NPD/molybdenum oxide) to be inthe range of 1 to 2.

Next, a result of a continuously lighting test of the light-emittingelements of the present embodiment is described. After thelight-emitting element manufactured as above is sealed under nitrogenatmosphere, the continuously lighting test is conducted at normaltemperature in the following way.

As is clear from FIG. 16, the current density of 26.75 mA/cm² isrequired when the light is emitted with the luminance of 3000 cd/m² inan initial state of the light-emitting element of the present invention.In this embodiment, the current of 26.75 mA/cm² keeps to be flowed for acertain period of time, and data are collected on the change of thevoltage required to flow the current of 26.75 mA/cm² over time and thechange of the luminance over time. FIGS. 18 and 19 show the collecteddata. In FIG. 18, the horizontal axis shows the passed time (hour),while the vertical axis shows the voltage (V) required for flowing thecurrent of 26.75 mA/cm². In FIG. 19, the horizontal axis shows thepassed time (hour), while the vertical axis shows the luminance (anyunit of measure). It is to be noted that the luminance (any unit ofmeasure) is a relative value to the initial luminance expressed byassuming that the initial luminance be 100. The relative value isobtained in such a way that the luminance at a particular time isdivided by the initial luminance and multiplied by 100.

It is to be understood from FIG. 18 that after 100 hours have passed,the voltage required for flowing the current having the current densityof 26.75 mA/cm² is only approximately 1 V higher than that in theinitial state. This indicates that the light-emitting element of thepresent invention is a superior element in which the increase in thedrive voltage over time is small.

In the light-emitting elements shown in Embodiments 1 and 2, layersserving as the hole-injecting layer, the hole-transporting layer, theelectron-transporting layer, and the like are formed in addition to thelayer serving as the light-emitting layer. However, these layers are notalways necessary. Further, in Embodiments 1 and 2, after the layerserving as the light-emitting layer is formed, the electron-generatinglayer is formed, and then the hole-generating layer is formed. However,the manufacturing method of the light-emitting element according to thepresent invention is not limited to this. For example, after thehole-generating layer is formed, the electron-generating layer may beformed, and then the layer serving as the light-emitting layer may beformed. The present application is based on Japanese PriorityApplication No. 2004-288972 filed on Sep. 30, 2004 with the JapanesePatent Office, the entire contents of which are hereby incorporated byreference.

EXPLANATION OF REFERENCE

-   100: an insulator, 101: a first electrode, 102: a layer, 103: a    layer, 104: a light emitting layer, 105: a layer, 106: a second    electrode, 107: a layer, 50: a substrate, 51 a: a base insulating    layer, 51 b: a base insulating layer, 52: a semiconductor layer, 53:    a gate insulating layer, 54: a gate electrode, 59: an insulating    film (hydride film), 60: an interlayer insulating layer, 61 a:    connection portion, 61 b: a wiring, 63: an interlayer insulating    layer, 65: a bank, 66: a light-emitting stack, 70: a thin film    transistor, 88: a resin, 89: a desiccant, 90: a polarizing plate,    91: a protective film, 93: a light emitting element, 94: a counter    substrate, 900: an interlayer insulating layer, 901: a thin film    transistor, 4001: a substrate, 4002: a pixel area, 4003: a signal    line driver circuit, 4004: a scan line driver circuit, 4005: a    sealant, 4006: a counter substrate, 4007: an filler, 4008: a thin    film transistor, 4010: a thin film transistor, 4011: a light    emitting element, 4014: a wiring, 4015 a: a wiring, 4015 b: a    wiring, 4016: a connection terminal, 4018: a flexible printed    circuit (an FPC), 4019: an anisotropic conductive film, 2001: a    case, 2003: a display portion, 2004: a speaker area, 2101: a main    body, 2102: a case, 2103: a display portion, 2104: a voice input    area, 2105: a sound reproducer, 2106: operation keys, 2108: an    antenna, 2201: a main body, 2202: a case, 2203: a display portion,    2204: a keyboard, 2205: an external connection portion, 2206: a    pointing mouse, 2301: a main body, 2302: a display portion, 2303: a    switch, 2304: operation keys, 2305: an IR port, 2401: a case, 2402:    a display portion, 2403: a speaker area, 2404: operation keys, 2405:    an recording medium insert portion, 1401: a switching TFT, 1402: a    capacitor, 1403: a driving TFT, 1404: a current control TFT, 1405: a    light emitting element, 1406: a TFT, 1410: a signal line, 1411: a    power source line, 1412: a power source line, 1414: a scan line,    1415: a scan line, 1500: a pixel area, 1561: a diode, 1554: a    wiring, 1555: a wiring.

1. A light emitting element comprising: a first electrode and a secondelectrode; a first layer and a second layer each generating holes; athird layer containing a light emitting material; and a fourth layergenerating electrons, wherein the first layer is in direct contact withthe first electrode, the second layer is in direct contact with thesecond electrode, the third layer is provided between the firstelectrode and the second electrode with the first layer and the secondlayer respectively therebetween, and the fourth layer is providedbetween the third layer and the second layer.
 2. A light emittingelement according to claim 1, wherein a thickness of each of the firstlayer and the second layer is 30 nm to 1 μm.
 3. A light emitting elementaccording to claim 1, wherein a thickness of the second layer is 50% to150% of a thickness of the first layer, and the thickness of the firstlayer is 50% to 150% of the thickness of the second layer.
 4. A lightemitting element according to claim 1, wherein t voltage is applied soas to make the light emitting element emit light, the electrode appliedwith higher potential is the first electrode, and the electrode appliedwith lower potential is the second electrode.
 5. A light emittingelement comprising: a first electrode and a second electrode; a firstlayer and a second layer each containing a P-type semiconductor; a thirdlayer containing a light emitting material; and a fourth layercontaining an N-type semiconductor, wherein the first layer is in directcontact with the first electrode, the second layer is in direct contactwith the second electrode, the third layer is provided between the firstelectrode and the second electrode with the first layer and the secondlayer respectively therebetween, and the fourth layer is providedbetween the third layer and the second layer.
 6. A light emittingelement according to claim 5, wherein the P-type semiconductor is ametal oxide.
 7. A light emitting element according to claim 5, whereinthe P-type semiconductor is one or more compounds selected from thegroup consisting of vanadium oxide, molybdenum oxide, cobalt oxide, andnickel oxide.
 8. A light emitting element according to claim 5, whereinthe N-type semiconductor is a metal oxide.
 9. A light emitting elementaccording to claim 5, wherein the N-type semiconductor is one or morecompounds selected from the group consisting of zinc oxide, zincsulfide, zinc selenide, and titanium oxide.
 10. A light emitting elementaccording to claim 5, wherein a thickness of each of the first layer andthe second layer is 30 nm to 1 μm.
 11. A light emitting elementaccording to claim 5, wherein a thickness of the second layer is 50% to150% of a thickness of the first layer, and the thickness of the firstlayer is 50% to 150% of the thickness of the second layer.
 12. A lightemitting element according to claim 5, wherein t voltage is applied soas to make the light emitting element emit light, the electrode appliedwith higher potential is the first electrode, and the electrode appliedwith lower potential is the second electrode.
 13. A light emittingelement according to claim 1, wherein the light emitting element isincorporated in one selected from the group consisting of a television,a mobile phone, a computer, and a game machine.
 14. A light emittingelement according to claim 5, wherein the light emitting element isincorporated in one selected from the group consisting of a television,a mobile phone, a computer, and a game machine.
 15. A light emittingelement comprising: a first electrode over a substrate; a first layergenerating holes over and in direct contact with the first electrode; athird layer containing a light emitting material over the first layer; afourth layer generating electrons over the third layer; a second layergenerating holes over the fourth layer; and a second electrode over andin direct contact with the second layer.
 16. A light emitting elementaccording to claim 15, wherein a thickness of each of the first layerand the second layer is 30 nm to 1 μm.
 17. A light emitting elementaccording to claim 15, wherein a thickness of the second layer is 50% to150% of a thickness of the first layer, and the thickness of the firstlayer is 50% to 150% of the thickness of the second layer.
 18. A lightemitting element according to claim 15, wherein t voltage is applied soas to make the light emitting element emit light, the electrode appliedwith higher potential is the first electrode, and the electrode appliedwith lower potential is the second electrode.
 19. A Tight emittingelement according to claim 15, wherein the light emitting element isincorporated in one selected from the group consisting of a television,a mobile phone, a computer, and a game machine.
 20. A light emittingelement comprising: a first electrode over a substrate; a first layercontaining a P-type semiconductor over and in direct contact with thefirst electrode; a third layer containing a light emitting material overthe first layer; a fourth layer containing an N-type semiconductor overthe third layer; a second layer containing a P-type semiconductor overthe fourth layer; and a second electrode over and in direct contact withthe second layer.
 21. A light emitting element according to claim 20,wherein the P-type semiconductor is a metal oxide.
 22. A light emittingelement according to claim 20, wherein the P-type semiconductor is oneor more compounds selected from the group consisting of vanadium oxide,molybdenum oxide, cobalt oxide, and nickel oxide.
 23. A light emittingelement according to claim 20, wherein the N-type semiconductor is ametal oxide.
 24. A light emitting element according to claim 20, whereinthe N-type semiconductor is one or more compounds selected from thegroup consisting of zinc oxide, zinc sulfide, zinc selenide, andtitanium oxide.
 25. A light emitting element according to claim 20,wherein a thickness of each of the first layer and the second layer is30 nm to 1 μm.
 26. A light emitting element according to claim 20,wherein a thickness of the second layer is 50% to 150% of a thickness ofthe first layer, and the thickness of the first layer is 50% to 150% ofthe thickness of the second layer.
 27. A light emitting elementaccording to claim 20, wherein t voltage is applied so as to make thelight emitting element emit light, the electrode applied with higherpotential is the first electrode, and the electrode applied with lowerpotential is the second electrode.
 28. A light emitting elementaccording to claim 20, wherein the light emitting element isincorporated in one selected from the group consisting of a television,a mobile phone, a computer, and a game machine.